1. Field of the Invention
This invention relates to improving the strength of yarn spun from anisotropic melt-forming polyesters.
2. Description of the Prior Art
A class of wholly aromatic polyesters that form optically anisotropic melts from which oriented filaments can be melt spun has been described in Schaefgen U.S. Pat. No. 4,118,372. Most polyesters which are disclosed in this patent are derived primarily from para-oriented dihydric phenols and para-oriented dicarboxylic acids. U.S. Pat. No. 4,083,829 discloses polyesters consisting essentially of p-oxybenzoyl moieties, 2,6-dicarbonylnaphthalene moieties and various other moieties. These are also said to exhibit anisotropy in the melt. Still other anisotropic melt-forming polyesters are disclosed in U.S. Pat. No. 4,153,779 and in many other patents and publications. These polymers are also described as liquid crystal or thermotropic polymers.
It has been taught by Luise U.S. Pat. No. 4,183,895 that yarn which has been spun from anisotropic melt-forming polyesters may be strengthened by heat-treatment and this has been acknowledged in U.S. Pat. No. 4,083,829 and in other patent literature. Quite often it is found that the yarns must be heat-treated for extended periods in order to achieve significant improvement in tenacity.
The use of alkali metal salts as accelerators for the heat-strengthening process is taught in Eskridge et al U.S. Pat. No. 4,424,184. According to the process of that patent, the yarns are coated with small amounts of the salts prior to heat-strengthening. By virtue of the present invention, one can enhance the effect of the accelerator.